1. Field of the Invention
This invention relates to a sheet feed tractor, and particularly to a lid structure for such sheet feed tractor.
2. Description of the Related Art
In a known sheet feed tractor, a lid is used for confining a paper sheet on a conveyer belt. The lid is pivotally mounted on a frame by means of a pair of hinges which extend in side by side relationship from the lid and from the frame. The lid hinges have pins while the frame hinges have holes. Both the lid and frame are connected by a spring. As shown in FIGS. 12 and 13 of the accompanying drawings, a hinge pin 25 of a lid 24 is inserted into a bearing hole 23 of a hinge 22 of a frame 21. In another sheet feed tractor shown in FIG. 14, a groove 33 which is open upwardly is formed at an end of a hinge 32 of a frame 31 so that a hinge pin 35 of a lid 34 is inserted into the groove 33.
When the lid is in an open or closed position, an edge of the lid rests upon stops of the frame hinges. The lid is biased to its respective positions by a tension spring.
When sheet feeding is performed, the lid is in a closed position with a space over the paper feeding plane, confining the paper sheet on the conveyer belt. Specifically, the lid remains parallel with the paper feeding plane. On the contrary, the open position of the lid varies according to a layout of a printer, or specifications of the sheet feed tractor or the printer.
The lid is usually self holding in a neutral position of 45 degrees with respect to the sheet feeding plane (where the lid is movable in any directions since there is no spring bias, theoretically). To close the lid, firstly the lid is moved with a flick of the finger to the neutral position of 45 degrees, then being closed by the force of the tension spring when the finger is released from the lid.
When opening, the lid is moved up manually until it passes over the neutral position and the outer edge of the lid reaches the stops on the frame hinge. In this case, an excessive force would sometimes be applied to the lid. The lid would be subject to shocks when something happens to strike the lid in the open position. Since the lid and the frame are made of plastics and are not strong enough, they would be deformed or damaged at their weak positions by such shocks. In such case, the sheet feed tractor would be somewhat distorted, thereby reducing sheet feeding efficiency. In a worst case, the sheet feed tractor has to be replaced, thereby causing printing to be interrupted, leading to loss of work efficiency.